Over the past forty years or so there has been an increasing realisation of the need to replace fossil fuels with more secure sustainable energy sources. The new energy supply must also have low environmental impact, be highly efficient and be easy to use and cost effective to produce. To this end, solar energy is seen as one of the most promising technologies, nevertheless, the high cost of manufacturing devices that capture solar energy, including high material costs, has historically hindered its widespread use.
Every solid has its own characteristic energy-band structure which determines a wide range of electrical characteristics. Electrons are able to transition from one energy band to another, but each transition requires a specific minimum energy and the amount of energy required will be different for different materials. The electrons acquire the energy needed for the transition by absorbing either a phonon (heat) or a photon (light). The term “band gap” refers to the energy difference range in a solid where no electron states can exist, and generally means the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band. The efficiency of a material used in a photovoltaic device, such as a solar cell, under normal sunlight conditions is a function of the band gap for that material. If the band gap is too high, most daylight photons cannot be absorbed; if it is too low, then most photons have much more energy than necessary to excite electrons across the band gap, and the rest will be wasted. The Shockley-Queisser limit refers to the theoretical maximum amount of electrical energy that can be extracted per photon of incoming light and is about 1.34 eV. The focus of much of the recent work on photovoltaic devices has been the quest for materials which have a band gap as close as possible to this maximum.
One class of photovoltaic materials that has attracted significant interest has been the hybrid organic-inorganic halide perovskites. Materials of this type form an ABX3 crystal structure which has been found to show a favourable band gap, a high absorption coefficient and long diffusion lengths, making such compounds ideal as an absorber in photovoltaic devices. Early examples of hybrid organic-inorganic metal halide perovskite materials are reported by Kojima, A et al. (2009) Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc., 131(17), pp. 6050-6051 in which such perovskites were used as the sensitizer in liquid electrolyte based photoelectrochemical cells. Kojima et al report that a highest obtained solar energy conversion efficiency (or power energy conversion efficiency, PCE) of 3.8%, although in this system the perovskite absorbers decayed rapidly and the cells dropped in performance after only 10 minutes.
Subsequently, Lee, M et al, (2012) Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science, 338(6107), pp. 643-647 reported a “meso-superstructured solar cell” in which the liquid electrolyte was replaced with a solid-state hole conductor (or hole-transporting material, HTM), spiro-MeOTAD. Lee et al reported a significant increase in the conversion efficiency achieved, whilst also achieving greatly improved cell stability as a result of avoiding the use of a liquid solvent. In the examples described, CH3NH3PbI3 perovskite nanoparticles assume the role of the sensitizer within the photovoltaic cell, injecting electrons into a mesoscopic TiO2 scaffold and holes into the solid-state HTM. Both the TiO2 and the HTM act as selective contacts through which the charge carriers produced by photoexcitation of the perovskite nanoparticles are extracted.
Further work described in WO2013/171517 disclosed how the use of mixed-anion perovskites as a sensitizer/absorber in photovoltaic devices, instead of single-anion perovskites, results in more stable and highly efficient photovoltaic devices. In particular, this document discloses that the superior stability of the mixed-anion perovskites is highlighted by the finding that the devices exhibit negligible colour bleaching during the device fabrication process, whilst also exhibiting full sun power conversion efficiency of over 10%. In comparison, equivalent single-anion perovskites are relatively unstable, with bleaching occurring quickly when fabricating films from the single halide perovskites in ambient conditions.
More recently, WO2014/045021 described planar heterojunction (PHJ) photovoltaic devices comprising a thin film of a photoactive perovskite absorber disposed between n-type (electron transporting) and p-type (hole transporting) layers. Unexpectedly it was found that good device efficiencies could be obtained by using a compact (i.e. without effective/open porosity) thin film of the photoactive perovskite, as opposed to the requirement of a mesoporous composite, demonstrating that perovskite absorbers can function at high efficiencies in simplified device architectures.
Recently some of research into the application of perovskites in photovoltaic devices has focussed on the potential of these materials to boost the performance of conventional silicon-based solar cells by combining them with a perovskite-based cell in a tandem/multi-junction arrangement. In this regard, a multi-junction photovoltaic device comprises multiple separate sub-cells (i.e. each with their own photoactive region) that are “stacked” on top of each other and that together convert more of the solar spectrum into electricity thereby increasing the overall efficiency of the device. To do so, each photoactive region of each sub-cell is selected so that the band gap of the sub-cell ensures that it will efficiently absorbs photons from a specific segment of the solar spectrum. This has two important advantages over conventional single-junction photovoltaic devices. Firstly the combination of multiple photoactive regions/sub-cells with different band gaps ensures that a wider range of incident photons can be absorbed by a multi-junction device, and secondly each photoactive region/sub-cell will be more effective at extracting energy from the photons within the relevant part of the spectrum. In particular, the lowest band gap of a multi-junction photovoltaic device will be lower than that of a typical single junction device, such that a multi-junction device will be able to absorb photons that possess less energy than those that can be absorbed by a single junction device. Furthermore, for those photons that would be absorbed by both a multi-junction device and a single junction device, the multi-junction device will absorb those photons more efficiently, as having band gaps closer to the photon energy reduces thermalization losses.
In a multi-junction device, the top photoactive region/sub-cell in the stack has the highest band gap, with the band gap of the lower photoactive regions/sub-cells reducing towards the bottom of the device. This arrangement maximizes photon energy extraction as the top photoactive region/sub-cell absorbs the highest energy photons whilst allowing the transmission of photons with less energy. Each subsequent photoactive region/sub-cell then extracts energy from photons closest to its band gap thereby minimizing thermalization losses. The bottom photoactive region/sub-cell then absorbs all remaining photons with energy above its band gap. When designing multi-junction cells it is therefore important to choose photoactive regions/sub-cells with the right bandgaps in order to optimise harvesting of the solar spectrum. In this regard, for a tandem photovoltaic device that comprises two photoactive regions/sub-cells, a top photoactive region/sub-cell and a bottom photoactive region/sub-cell, it has been shown that the bottom photoactive region/sub-cell should have a band gap of around 1.1 eV whilst the top photoactive region/sub-cell should have a band gap of around 1.7 eV (Coutts, T et al, (2002). Modeled performance of polycrystalline thin-film tandem solar cells. Progress in Photovoltaics: Research and Applications, 10(3), pp. 195-203).
Consequently, there has been interest in developing hybrid organic-inorganic perovskite solar cells for use in tandem photovoltaic devices given that the band gap of these perovskite materials can be tuned from around 1.5 eV to over 2 eV by varying the halide composition of organometal halide perovskites (Noh, J. et al, (2013). Chemical Management for Colorful, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells. Nano Letters, p. 130321112645008). However, to date the stability of the hybrid organic-inorganic perovskite has proven to be a hurdle to their potential use in commercially viable photovoltaic devices. In particular, whilst introducing mixed halides into hybrid perovskite compositions allows for band gap tuning, these mixed halide hybrid perovskites are typically even less stable than the comparable single halide perovskites.